Mixing is a term applied to actions which reduce non-uniformities of materials involved. Such materials can be liquids, solids or gases, and the non-uniformities in such materials can occur in various properties, such as color, density, temperature, etc. The quality of mixing can be described by two characteristics--scale ("S") and intensity ("I"). The scale of mixing is the average distance between the centers of maximum difference in a given property of the mixture, and intensity is the variation in a given property of the mixture.
The terms "S" and "I" are easily understood by the following illustrations. Assume that in a shallow dish of white paint, a number of randomly dropped dollops of viscous black paint have been applied. Where all black paint within a dollop resides, the intensity "I" is one hundred percent. In regions of white paint the intensity is zero percent. The distance between the center of the black dollop and an adjacent white region is called the scale of mixing.
If the dish of paint were allowed to sit untouched, the demarkation between black and white would begin to blur as the peak or one hundred percent intensity of the black paint diminishes, and the zero intensity of the white paint rises. Finally, when enough time has passed, the intensity variation will asymptote to zero, and a uniformly gray paint mixture will result. Obviously, the smaller the scale of mixing, the more rapidly will the intensity variation asymptote to zero. Conversely, the higher the molecular diffusion, the larger the scale of mixing can be in achieving a given degree of mixedness for a given time period. Generally speaking, the higher the viscosity of a fluid, the lower will be its rate of molecular diffusion in any given solvent.
As design goals in producing the mixture of the present invention, it was the intent to reduce the scale of mixing rapidly, and thus promote a rapid drop in intensity.
The principles outlined above have particular application in the mixing of special polymers which are used in water treatment applications. These polymers are usually supplied having viscosities that can range from a few thousand centipoise to the order of one million centipoise. The polymers are generally diluted on site to save shipping costs and injected and mixed with the water to be treated as they cause particulates in water to agglomerate to form what is called "floc,"which can then be filtered.
Obviously, such high viscosity polymers are difficult to dilute on site. The conventional mechanical mixing approach, consisting of a motor driven paddle or blade in a tank, is clumsy, inefficient, and ineffective. Large lumps of undiluted polymer can circulate for hours or even days without being dissolved into solution. In addition, the very high shear rates associated with the tips of the blades can damage shear-sensitive polymers by breaking up the long chain polymers and reducing the flocculation efficiency. This is particularly true for emulsion polymers.
Even though such special polymers used in water treatment applications are introduced to, for example, ten times their own volume of water, the mixture will have a much higher viscosity than the original, undiluted matter--often ten to fifty times higher. Typical dilution ratios are 200:1. In examining this problem, it became obvious that an appropriate mixing system would be one which would break up the water/polymer elements into very small components so as to achieve a minimum scale of mixing. It was also recognized that the appropriate mixing system should be one which could provide for controlled shearing to cause a smearing of the elements together. This aids in molecular diffusion by increasing the interfacial area and by reducing interfacial thickness. It was obviously a design goal to accomplish this result in the shortest amount of time, preferably in the order of one second or less.
Parent U.S. application Ser. No. 34,672 disclosed a device for the mixing of two or more liquids which was particularly effective in mixing such things as those water treatment polymers discussed previously. The device in the prior application consisted of the use of a hollow shaft connected to a drive motor which would cause the shaft to rotate. A shell body was employed to house the rotatable shaft, such shell body having inlets for the liquids to be mixed approximate one end thereof. Slotted grooves were configured within the hollow shaft for receiving the liquids to be mixed from the inlets located within the shell body. A narrow annular gap region was formed between the outer surface of the hollow shaft and the inner surface of the shell body. A first set of holes was configured in the hollow shaft located downstream of the narrow annular gap region for the introduction of the liquids into the interior of the hollow shaft. A second set of grooves configured in the hollow shaft located downstream from the first set of holes was used for dispensing the liquids from the interior of the hollow shaft and through the shell body.
Although the invention disclosed and claimed in parent U.S. application Ser. No. 34,672 represented a marked advance in the art, the channeling of the liquids to be mixed through the annular gap region and within the hollow shaft resulted in significant pressure drops being measured across the mixing device. It has now been found that a similar mixing device can be configured displaying virtually all of the beneficial characteristics of the device disclosed in the parent application while exhibiting significantly reduced pressure drops.